The users of a telecommunications service, such as those in call centers, may sometimes wish to present a customized telephone number to the recipients of their calls. For example, the operator of a call center may be interested in having the recipient of a call see a recognizable or useful number, such as a “1-800” number, rather than a number associated with the particular phone that was used to initiate the call. This might be accomplished by simply substituting the desired external-facing number for the original telephone number in the call message information. However, particularly in the context of telecommunications systems operating on general-purpose networks, controlling the external-facing telephone number may conflict with other features of the telecommunications system. For example, substituting a customized external-facing number for an original number may interfere with billing processing which requires access to the original non-customized number. Likewise, fraud protection features of the system may require access to the non-customized number.